1. Field of the Invention
This invention relates to a waveguide type wavelength domain optical switch.
2. Description of the Related Art
FIG. 10 shows a conventional wavelength domain optical switch (See US patent publication No. 2006/67611 A1).
As shown in FIG. 10, the wavelength domain optical switch 600 is composed of input/output optical fibers 601 to 606, a collimating lens array 610, a Wollaston prism 615 (composed of two triangular prisms 616, 617) for allowing independence of characteristics between horizontal polarization (y-polarization) and vertical polarization (x-polarization), a birefringent plate 620 for zeroing a phase difference between the horizontal polarization and the vertical polarization, a half-wave plate 625 (where only 626 is a half-wave plate and 627 has no effect on polarization), a concave mirror 630, a cylindrical lens 635, a grating 642 with a wedged prism 641, a prism 646 for bending light in perpendicular direction, and an LCOS SLM (liquid crystal on silicon spatial light modulator).
In the wavelength domain optical switch 600, for example, a wavelength-multiplex beam outputted from the input/output optical fiber 602 is split into two light beams with a same polarization direction, reflected by the concave mirror 630, and inputted to the grating 642. The grating 642 demultiplexes the incident beams into light beams with different wavelengths which are inputted into the LCOS SLM 645. The LCOS SLM 645 conducts a phase modulation such that the light beams with different wavelengths are collected at a position of an optical fiber for outputting them.
FIGS. 11A and 11B show a conventional waveguide type wavelength selective optical switch using an MEMS (micro electro mechanical system) micro mirror (See U.S. Pat. No. 7,088,882).
As shown in FIGS. 11A and 11B, the waveguide type wavelength selective optical switch 100 is structured such that five waveguide spectrometers 102 are disposed on one substrate 101 and the five substrates 101 are stacked. The waveguide type wavelength selective optical switch 100 allows the MEMS micro mirror 103 to reflect light beam at a predetermined angle. In FIGS. 11A and 11B, 104 is a micro lens array for collecting light beam at respective input/output ends of the waveguide spectrometers 102, and 105 is a collimating lens.
However, the wavelength domain optical switch in FIG. 10 has problems as below.
First, since it uses the bulk grating (i.e., the grating with the wedged prism), it is difficult to decrease the dimensions of the entire grating although only the one component is advantageously needed for demultiplexing.
Second, it needs to use the complicated optical system such as the birefringent plate 620 for zeroing a phase difference between the horizontal polarization (y-polarization) and the vertical polarization (x-polarization), and the half-wave plate 625 (where only 626 is a half-wave plate and 627 has no effect on polarization). This is because it is necessary to eliminate the polarization dependency caused by using the bulk grating with large polarization dependency and the conventional optical phase modulation cell LCOS SLM 645.
Third, in constructing the complicated optical system, the cost of the optical parts used increases along with the assembly cost. Thus, it is difficult to reduce the manufacturing cost.
Fourth, it is difficult to arrange a measure for monitoring a wavelength and power of optical signal which is necessary for the optical network. Therefore, an additional module is newly needed for monitoring.
The waveguide type wavelength selective optical switch using the MEMS micro mirror in FIGS. 11A and 11B has problems as below.
First, although the MEMS micro mirror allows the structure that the plural waveguide spectrometers are in parallel disposed on one substrate due to the large reflection angle of the MEMS mirror, this structure cannot be applied to a wavelength domain optical switch using the LCOS SLM. This is because it is necessary to have a width of 100 mm or so in order to dispose the waveguide type wavelength selective optical switch using the MEMS micro mirror on the substrate. In this case, the LCOS SLM cannot reflect all light beams since it has only a small reflection angle. Thus, such an optical switch must significantly deteriorate in switching performance.
Second, the lens array for collecting light beams in the perpendicular direction disposed corresponding to the respective waveguide spectrometers causes the problems that the assembly time increases since the adjustment of optical axis between the lens array and the respective waveguide spectrometers is very strict to conduct, and that an expensive aspherical lens is needed for reducing aberration in the lens array. These problems become more serious according as the lens decreases in size. Thus, it is very difficult to downsize the optical switch.
Third, since the waveguide spectrometers are horizontally disposed without being inclined to input/output light beam, reflection loss can be caused at the end of the respective optical parts. Therefore, the switching characteristics may lower.
Fourth, where the plural substrates with the waveguide spectrometers are vertically stacked to increase the number of outputs, they cannot be stacked so closely to each other since the micro lens for reducing aberration disposed corresponding to each output is limited in downsizing. Thus, the optical switch is very difficult to downsize for further increasing the number of outputs.